Amino acids as corrosion inhibitors for copper in acidic medium: Experimental and theoretical study

Experimental electrochemical methods combined with quantum chemical calculations and molecular dynamics simulations were emplyed to investigate the possibility for the use of various amino acids as green corrosion inhibitors for copper in 0.5 M HCl solution. Among the eleven studied amino acids, cysteine achieved the highest inhibitor effectiveness, reaching 52 % at a concentration of 10 mM. The other amino acids achieved an effectiveness of less than 25 %; some of them even acted as corrosion accelerators. Based on the experimental results, the theoretical calculations and simulations were focused on cysteine and alanine. The electronic and reactivity parameters of their protonated forms in an electrical double layer were evaluated by density functional calculations. In addition, molecular dynamic simulations were introduced to follow the adsorption behaviour of these two amino acids at the Cu(111) surface in the electrolyte solution. The results indicate that the orientation of both molecules was nearly parallel to the surface except for the ammonium group, which was directed away from the surface. Therefore, as the orientation of the cysteine and alanine molecules at the surface were similar, the thiol functional group is responsible for the superior inhibition efficiency of cysteine.